The high mechanical strength properties of ceramic fibers and their resistance to chemical attack have made ceramic fibers attractive for use in a variety of applications, such as reinforcement materials for aircraft engine parts and aircraft structures, as well as combustors and radiative burning parts. However, to retain their desirable properties in oxidizing environments, it is necessary to coat the fibers with an interface coating to provide a surface which will keep the fibers distinct from the matrix with which the fibers form a composite. Stated otherwise, the fibers are intended to reinforce the matrix and, therefore, cannot diffuse into the matrix during processing. It is the ability of the fiber to be stressed independently of the matrix which imparts desirable ductility and fracture toughness to ceramic fiber composites. To maintain the fibers distinct from the matrix, the interface coating should function as a lubricant and exhibit only a weak mechanical bond to the matrix.
Currently, pyrocarbon and boron nitride are the most commonly used interface coatings for ceramic fibers in fiber-reinforced ceramic composites. Such coatings are chemically distinct from, and react little with, the principal reinforcing fibers, which are typically composed of silicon carbide, silicon nitride, aluminum oxides, and various metal silicates, and matrices, such as glass, metal oxide, silicon carbide, and silicon nitride. Pyrocarbon and boron nitride exhibit weak mechanical bonding to the matrix when applied to the fiber. However, both of these interface coatings are susceptible to relatively rapid oxidation at temperatures exceeding 1000.degree. C., so that their use is limited to ceramic composites designed to operate below 1000.degree. C. There is no commercially satisfactory interface coating suitable for sustained use above 1000.degree. C. in oxidizing atmospheres, even though ceramic composites are generally regarded as refractory materials. Boron nitride coatings, which have a higher temperature capability than pyrocarbon coatings in oxidizing atmospheres, are also subject to attack by moisture.
Although ceramic fibers and matrices exist which retain their mechanical properties in oxidizing environments at temperatures exceeding 1000.degree. C., fiber-reinforced composites which can withstand the same conditions have not yet been developed for lack of a suitable interface coating. Accordingly, there is a need for an interface coating which performs well under these conditions.